How to debug the cacheable response?

hey all,

According to https://www.drupal.org/docs/8/api/responses/cacheableresponseinterface#d…, changing “http.response.debug_cacheability_headers” to true will show two new cache headers in drupal pages, but it’s not. I can never see the two headers as described at https://www.drupal.org/node/2222835.

     $c = Drupal::getContainer();

     $p = $c->getParameter(‘http.response.debug_cacheability_headers’);
     var_dump($p);

$p shows boolean true, but the two headers are not showing.

Drupal version: 


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